Competitive Adsorption Mechanism Study of CHClF2 and CHF3 in

Subscriber access provided by Kaohsiung Medical University

Article

Competitive adsorption mechanism study of CHClF2 and CHF3 in FAU zeolite Qiang Fu, Yingjie Qin, Donghui Zhang, and Yangyuan Han ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00874 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Competitive adsorption mechanism study of CHClF2 and CHF3 in FAU zeolite Qiang Fu, Yingjie Qin, Donghui Zhang,* Yangyuan Han Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

ABSTRACT Separating and recovering the CHF3 and CHClF2 greenhouse gases from the exhausted gases is quite important for avoiding adverse impacts to the environment. In synthesizing chlorofluorocarbons substitutes, Faujasite (FAU) is proved to be effective in separating the hydrofluorocarbons. The grand canonical ensemble Monte Carlo simulations were used without precedent in analyzing the competitive adsorption

mechanism

of

CHF3/CHClF2

in

Na Al Si O

(58Al),

Na Al Si O (88Al) and Na Al Si O (96Al) FAU zeolite model from infinite dilution to saturation adsorption, so as to explore the overall competitive relationship as the adsorption amount increases. As a result, it has been found that the sodium migration degree is affected by the guest-host effect in 58A1 model. However, the sodium migration is not discovered in 88Al and 96Al model with diversified CHF3 or CHClF2 loadings. The preferred binding site in all FAU zeolite model involves CHClF2 and CHF3 anchored by cations in site II and site III'. Adsorption enthalpy predicted is highly correlated upon adsorption site and geometry structure of CHClF2 and CHF3 in FAU zeolite. The CHF3 molecule is preferentially adsorbed via the short-range force of its

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

hydrogen center with the lattice oxygens of the FAU supercage. In addition, the CHF3 selectivity would be enhanced to a certain degree with ratios of Si/Al and adsorption temperatures being lowered. Through this computational study, the inherent competitive adsorption mechanism at the molecular level is clearly illustrated, thus providing an effective strategy of designing and screening adsorptive materials, so as to have a better separation and recovery of CHF3-CHClF2.

Keywords: Competitive adsorption mechanism, CHClF2 and CHF3 capture, adsorption site, adsorption heat, FAU zeolite

Corresponding author at: The Research Center of Chemical Engineering, School of Chemical Engineering and

Technology, Tianjin University, Tianjin 300072, China. Tel.: +086-022-27892097.

E-mail address: [email protected] (D.-H. Zhang).

Introduction Because it is closely related to the environmental problems about the HCFCs consuming the ozone layer and the elimination of chlorinated solvent residues in the polluted groundwater and soils, the performance of adsorbing hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) to faujasite type zeolites nowadays attracts

extensive

attention

worldwide.1

In

the

synthesis

process

of

chlorofluorocarbons (CFCs) substitutes, Faujasite has been proved of its separation reliability of HCFCs. Gray et al have studied the defluorination reaction of HFC-134

2


Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on the basic faujasite zeolites. 2 Their investigation found that CF2HCF2H molecules were binded more tightly to the zeolite than CF3CFH2 molecules based on solid state NMR analysis. Zeolites also used to be employed to separate HFCs mixtures from water.3 Lim et al. utilized the NMR and X-ray combination techniques to study the separation of HFCs isomers, through which, they found that a large amount of Na cations came out of site I’, went into the supercage and then the hexahonal prisms after adsorbing HFC-134 and HFC-134a through Na MAS Magic-Angle Spinning and in-situ X-ray powder diffraction technologies.4-6 Chatterjee et al. have used first principle methodology to investigate several CFCs adsorption structures and energy in FAU-type zeolites. They came to a conclusion that the host-guest interactions can be calculated based on the reactivity indexes of the active site. As a result, they succeeded in identifying a series of CFC active sites, selecting the zeolite clusters representing zeolite HY and NaY.7 In fluorocarbons, Chlorodifluoromethane (CHClF2) is the highest demand HCFCs widely used in air conditioner, commercial refrigerating and extruded polystyrene foams as well as the manufacture of pentafluoroethane and fluoropolymers.8-9 In manufacturing CHCIF2, the generation of Trifluoromethane (CHF3) is unable to be avoided. As a very effective greenhouse gas, according to the Kyoto Protocol, CHF3 has a potential of global warming of 14800 in the 100 year period and its life in the atmosphere is 270 years.10-14 To our knowledge, effective separation of CHClF2 and CHF3 mixtures have not been studied by using cationic

3



faujasites like NaX and NaY distinguished by their Si:Al ratios [Si:Al (X) < 1.5, Si:Al (Y) > 1.5]. Investigation of adsorption site and energy about CHClF2 and CHF3 on NaX or NaY is essential to understand the process of adsorption separation of CHClF2 and CHF3 mixtures. It is vital to understand many related processes from the scientific concept to the industrial application to carry out deep molecular level characterization to the adsorption phenomenon in the nanopores. In this respect, molecular simulation is a powerful tool, which can be used to conduct detailed research to the molecular arrangement of the confined fluid.15 Jaramillo et al have developed and performed a brand force field to simulate the dynamics of HFCs in faujasite-type zeolites at the same time. The results of their molecular dynamics simulations coincided with the test data of the adsorption heats and guest-host distances of HFC-134 and HFC-134a on NaX (Si:Al = 1.2) and Na-Y (Si:Al = 2.4).16 Zheng et al. studied the adsorption site of benzene on HY zeolite through the Monte Carlo (MC) simulations based on the force field, emphasized assessing the influence of the framework protons to the stability and distribution of adsorption sites.17 Torresknoop et al. introduced the screening of styrene and ethylbenzene under liquid conditions. They recommend separation according to the difference of saturated loading, because it has higher cost effectiveness and the most effective use of pore volume.18 But the potential adsorption mechanism of CHClF2 and CHF3 in FAU zeolites at the molecular level has not yet been comprehended. With above background we aim to rationalizing the mechanism of CHClF2 and CHF3 adsorption in FAU zeolite.

4


Page 4 of 29

Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In this paper, the grand canonical ensemble Monte Carlo simulation was firstly employed to analyze the CHF3/CHClF2 competitive adsorption mechanism of the FAU zeolite from the states of infinite dilution and saturation adsorption. The agreement adsorption isotherms of pure gases obtained through Grand Canonical Monte Carlo (GCMC) simulation as well as those from 273, 298 and 323 K experiments were used to validate the force field. Then, geometries of the adsorption sites and adsorption heats of FAU/CHClF2 as well as the CHF3 system were observed. The preferred binding site in FAU model involving CHClF2 and CHF3 anchored by cations at site II and site III' was investigated. It also discussed the intrinsic mechanism of CHCIF2 and CHF3 in FAU model adsorption process from the perspective of molecular levels, analyzed two practical influential factors, adsorption temperature as well as the Si/A1 rate of the zeolites.

Simulation and experiment Simulation models Based on the experimental single-crystal X-ray diffraction studies conducted by

Olson et al, the single cell dehydrated FAU zeolite models in the cell length of 25.099 Å were established.19 Their chemical composition Na AlSi O was considered with x = 58, 88, 96, labeled as 58Al, 88Al, 96Al, respectively. Just in this manner, the FAU zeolite models with Si:Al rated at 2.31, 1.18 and 1.0 were acquired. These constructed zeolite models were all constituted by eight supercages which were windows with 12 membered ring in the diameter of 7.4 Å. The supercages, which were 12.5 Å in diameter, were to occupy the adsorption of adsorbates.

5



The sodium ions are small enough to access the sodalite cages, but the bigger CHClF2 or CHF3 molecules are exclusively in the large cavities and not in the sodalite cages.20-21 They need to be artificially blocked. Therefore, we conducted artificial blockage to the solidate cages of zeolites which were difficult to be accessed in the whole simulation. Different crystallographic sites in Figure 1, which were respectively marked with I, I’, II, II’, III and III’ were preferentially occupied by the monovalent extra-framework sodium cations.22

Figure 1. Idealized location of extraframework Na cations in faujasite-type zeolites

Simulation details In order to simulate the adsorption, we kept the FAU framework rigid while the non-framework cations mobile. The adsorbates, CHClF2 and CHF3 molecules were constructed in the software Material Studio as a rigid tetrahedral molecule with five charged interaction sites, were shown in Figure 2. The following Eq (1) was used to model the non-bonding interactions of the guest molecules (CHClF2, CHF3 and sodium cations) to the host framework of the zeolite through Lennard-Jones (LJ) and

6


Page 6 of 29

Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Coulombic potentials: σij

12

σij

6

Ug-z = ∑i,j 4εij () + - ) + / + rij

where U5

6

rij

1

4πε0

∑i,j

qi q j rij

(1)

is the interaction between guest molecules and host, σij is the

collision diameter, εij is the potential well depth, rij indicates the distance between sites i and j, ε0 is the vacuum permittivity, qi refers to the atomic charge of site i. Table 1 displays LJ potential parameters of intermolecular interactions, which are used to adsorb the CHClF2 and CHF3 in FAU while table 2 demonstrates the atomic partial charges of CHF3 and CHClF2 sorbates. Furthermore, the rules of Lorentz−Berthelot mixture, such as: εij = (εii﹒εjj)1/2, σij = (σii + σjj)/2 were used to calculate the cross interactions with other molecules and frameworks. The following is the simulation scheme which has been implemented. First of all, CHClF2, CHF3 and zeolite models had their structures being optimized through the smart minizer method.23 Secondly, GCMC simulations were also acted as a calculation for CHClF2 and CHF3 adsorption isotherms in three constructed FAU zeolite models of 273, 298 and 323 K respectively. 1 × 107 equilibration and 1 × 107 production steps were included in the process, with each procedure being considered to try to move each single adsorbate for one time. The trial movement of each step was selected freely with fixed probabilities: 20% guest molecule translation, 20% for guest molecule rotation, 15% for guest molecule regrowth and 25% for guest molecule exchange (the total amount of creating and deleting procedures ratios with the equal weighing). Then in order to investigate the CHClF2 and CHF3 geometries of adsorption sites in FAU model employed, the MC 7



simulations were performed in the canonical ensemble (NVT) using the new force field at 298 K. In this regard, the following equation was used to calculate the adsorption energy on the basis of the isosteric heats of adsorption (Qst): Q 89 = RT − =

>?@AB @CDEFA G >HAB

I

J,K

(2)

in which, Eintra indicates intramolecular energy of adsorbate molecules (CHClF2, CHF3 molecules), Ead refers to the interactions summation between the above two kinds of adsorbates molecules (Eads-ads) and the interactions summation of all the adsorbates and the zeolites framework (Eads-zeo), Nad indeed indicates the overall adsorbates loadings (CHClF2, CHF3 molecules). The following equation was used to calculate the selectivity for CHF3 (S):

S=

LM /LO

PM /PO

(3)

in which, qi is the loadings of the species i and pi is its partial pressure.

8


Page 8 of 29

Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Lenard-Jones force field parameters on FAU zeolite

CVWXY 26 HVWXY 26 FVWXY 26 CVVRWXO 24 HVVRWXO FVVRWXO 24 ClVVRWXO 24 Na 25

OQR

OST

Na

CVWXY

HVWXY

FVWXY

3.26 71.72 2.80 48.55 2.96 48.36 3.25 54.45 2.80 48.55 2.97 49.94 3.24 106.93 3.40 23.00

3.26 71.72 2.80 48.55 2.96 48.36 3.25 54.45 2.80 48.55 2.97 49.94 3.24 106.93 3.40 23.00

3.05 52.58 2.59 35.59 2.75 35.45 3.04 39.92 2.59 35.59 2.76 36.61 3.03 78.39 2.584 50.27

3.52 55.00 3.06 37.23 3.22 37.08 3.51 41.75 3.06 37.23 3.23 38.30 3.49 81.99 3.05 52.58

3.06 37.23 2.60 25.20 2.76 25.10 3.05 28.26 2.60 25.20 2.77 25.92 3.04 55.50 2.59 35.59

3.22 37.08 2.76 25.10 2.92 25.00 3.21 28.15 2.76 25.10 2.93 25.82 3.19 55.28 2.75 35.45

σ(Å) value is the top entry; ε/kB (k) is the bottom entry Table 2. Atomic charges used to model the adsorption of CHF3 and CHClF2 in FAU element

CVWXY

HVWXY FVWXY

charges

26

0.30

26

0.06

26

CVVRWXO

HVVRWXO FVVRWXO

-0.12

27

0.204

27

-0.013

27

ClVVRWXO

-0.086

27

-0.019

28

2.05

28

1.75

Si

Al

OST

28

OQR

-1.025

28

-1.20

9



Page 10 of 29

Figure 2. Adsorbates model of (a) CHF3 and (b) CHClF2 molecules. Color code: gray, C; white, H; green, Cl; turquoise, F.

Results and discussion Adsorption performance of the single component and binary components The

sodium

ion-exchanged

zeolites

with

chemical

composition

of

Na Al Si O , Na Al Si O and Na Al Si O were employed in experiments to test adsorption isotherms. The average crystallites size of 1.0 µm were commercial product supplied by Shanghai Energy Chemical CO., LTD (China). The adsorption isotherms of pure CHClF2, CHF3 and equimolar CHF3/CHClF2 mixtures were calculated, so as to be compared with the experimental data, with an expectation to confirm the reliability of our force field (the calculation details for the adsorption isotherms of equimolar CHF3/CHClF2 mixed gases generated in experiments are listed in the supporting information). The adsorption isotherms of CHClF2 and CHF3 in FAU-type zeolites at diversified temperatures were calculated according to the different densities of Na+ cations. It is worth noting that the isotherms we calculated represented the isotherm shape and the saturation adsorptive capacity of the validation data set in the experiment as well. For instance, Figure 3 demonstrates the high consistency between the isotherms calculated from the pure CHClF2, CHF3 and equimolar CHF3/CHClF2 mixtures and the experimental data of 96Al FAU zeolite

10


Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


model at 273, 298 and 323 K respectively. First of all, from Figure 3, it is visible that all isotherms conform to the Langmuir isotherms and follow the physical adsorption rules, no matter the CHClF2 and CHF3 adsorbed jointly or independently. Secondly, with the increasing pressure, the adsorbates increased their adsorption amounts gradually and reached the saturated adsorptions in the end respectively. As to the pure component adsorption, as the pressure increases from 1 to 600kPa, the pure adsorption amounts of CHClF2 and CHF3 had dramatic increase, and approached to their saturated adsorptions of 39.3 and 68.2 molecule/unit cell at 298 K each ultimately, which further indicates that the saturated adsorption for CHF3 for pure component adsorption was 70% higher of that for CHClF2. Relatively speaking, binary CHClF2/CHF3 mixtures had different adsorption isotherms. In case the adsorption amount of CHClF2 and CHF3 was ready to be saturated, as the pressure increased from 1.0 to 600 kPa at 298 K, the load amount of CHF3 also had a dramatic growth from 23.38 to 68.79 molecule/unit cell. It is interesting that the CHClF2 isotherm in binary mixtures was also found to have a little increase of 2.54 molecule/unit cell in the same pressure scope. In the end, CHF3 and CHClF2 reached 68.79 and 2.82 molecule/unit cell respectively, which were indeed their own saturated adsorption loadings. This is because the curve slopes gradually approach to zero with the continuously rising pressure. The loadings of both CHF3 and CHClF2 further indicate that the two indeed had competitive adsorption relationships. It is visible that when CHClF2 was mixed with CHF3 molecules, it had the

11



saturated adsorption of less than 3.0 molecule/unit cell at 298 K. This adsorption amount was about 39.3 molecule/unit cell as CHClF2 adsorbed lonely, but 2.82 molecule/unit cell when CHClF2 and CHF3 mixed together. Therefore, when both CHClF2 and CHF3 absorbed in the FAU-type zeolite, the adsorption amount of CHClF2 had a significant decrease. Because CHF3 molecules were higher than CHClF2 in the saturation adsorption, we believe that the combined adsorption of them in 96Al FAU zeolites was more competitive than the single CHClF2 .

Figure 3. Adsorption isotherms for CHClF2 and CHF3 absorbed solely and together in 96Al FAU zeolite model at 273, 298, 323 K.

Cation site occupancies and adsorption site geometry Based on the previous study of Calero, the existence of the adsorbates might have a profound influence to the position of Na cations.28 However, the adsorption

12


Page 12 of 29

Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


performance of adsorbates is greatly affected because of the existing Na cations in faujasites.16 Therefore, with an expectation to study the influence of the extraframework cations to the adsorption performance, we firstly simulated the zeolites free of any adsorbates at 298k, so as to contrast the Na force field with the experimental data we can obtain. Afterwards, we carried out MC simulation to three FAU zeolite models in the canonical ensemble at 298K, with the estimated pressure reaching as high as 1000kPa. After the simulations implementation for the FAU zeolites without any adsorbate, we find that the sodium cation occupancies are in full accord with the experiment19, 29-33

and simulation data

34-35

listed in Table 3. As to 58Al model, the sodium cations

are mainly located at sites I, I’, and II in simulations. As to models 88Al and 96Al, the simulation results well coincide with the experimental data, in which, sites I’, II, and III’ are the main locations of cations. The coincidence between simulations and experiments is proved to be significant once again in the scope of the experimental error (no more than 5%). Then, our force field is even more corroborated. Afterwards, we continued to simulate the CHF3 and CHClF2 adsorption in canonical ensemble. Table 4 shows the sodium cation distributions in sodium FAU zeolite with CHF3 loading at 298 K (Table S1 shows the the sodium cation distributions in sodium FAU zeolite with CHClF2 loading). The sodium cations are redistributed obviously through adsorbing CHF3 or CHClF2. In 58Al FAU model, the quantity at site II’ and site III’ as well as the sites in the supercages which are not localized are improved by the CHF3 or CHClF2 adsorption at the expense of site I’ and site II. With the effect of CHF3 or

13



Page 14 of 29

CHClF2 adsorption, a smaller sodium is induced from sites I’ and II to sites II’ and III’. In 88Al and 96Al FAU models, the simulation results demonstrate that sodium has no migration at sites II and III’ with different CHF3 or CHClF2 loadings. Table 3. Sodium cation distributions (sodium cations per site per Unit Cell) in bare sodium FAU zeolite at 298 K Model

I

I’

this work Van Dun et al32 Fitch et al30 Newsam et al31

7.3 7.04 7.1 10

15.7 13.76 18.6 12

this work Olson19 Vitale et al33 Feuerstein et al29

3 2.9 3.1 4

26 29.1 30.08 24

3

32 32 32 29

this work Cheetham et al33 Boutin et al34 Auerbach et al35

II 58Al model 29.6 29.44 32.2 32 88Al model 31 31 31.07 29 96Al model 32 32 32 32

14


II’

III’

2

3.4 3.76 4 28 29.8 23.75 31 32 32 32 32

Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Sodium cation distributions (sodium cations per site per Unit Cell) in CHF3-loaded sodium FAU zeolite at 298 K CHF3 pressure/kPa

I

I’

10 30 50 100 500 1000

7.3 7 7.0 6.0 7.0 7.0

15.7 15.8 13.5 8.0 4.8 4.6

10 30 50 100 500 1000

3 4.7 2 3.5 4 3.2

26 24.3 27 25.5 25 25.8

10 30 50 100 500 1000

4 0.5 1 2 1.7

32 28 31.5 31 30 30.3

II 58Al model 29.6 28.9 26.2 21.4 18.6 18.3 88Al model 31 31 31 31 31 31 96Al model 32 32 32 32 32 32

II’

III’

2 2.3 4 9.7 12.9 13

3.4 4 7.3 12.9 14.7 15.1 28 28 28 28 28 28 32 32 32 32 32 32

Geometric situations of CHF3 and CHClF2 inside the supercage of FAU framework were aslo investigated by MC simulation. The picture in Figure 4a and 4b clearly shows the interaction of CHF3 molecule with Na+ (site II and III’) and Na+···F in 96Al FAU zeolite in the low loading state. This observation has aslo been found in 58Al and 88 Al FAU model systems (The results are showed in Figure S3-S6). With the increasing loading (pictures c and d in Figure 4), the adsorbate continues to interact with Na+ (site II and III’) increasingly solvated by the CHF3 molecules around. Similar adsorption site property is observed for CHClF2 molecule in 96Al FAU zeolite model, which is shown in Figure 5.

15



According to our simulations, it can be predicted that a small amount of site III’ cations exist in 58Al zeolite in the circumstance of low CHF3 or CHClF2 loadings, with CHF3 or CHClF2 being adsorbed at this preferred binding site. With the increasing CHF3 or CHClF2 loadings, cations migrate to the supercage, so as to provide more preferred adsorption sites, which are still insufficient for each adsorbate. This explains the predicted extraframework sodium cations migration as loadings of CHF3 (CHClF2) in 58Al FAU zeolite shown in Table 4 (Table S1). Furthermore, Figure 4a and Figure 5a represent typical binding geometry of CHF3 and CHClF2 adsorbed in the supercage of 96Al FAU zeolite. From Figure 4a, the Na-F distance in the optimized geometry is around 2.47 Å and 2.32 Å, the distance between the hydrogen and framework oxygen is 2.68 Å. Figure 5a displays the interaction between CHClF2 and FAU zeolite. It shows that the Na-F distance increased and then gradually stabilized at 3.49 Å while the Na–Cl distance gradually fixed at 5.46 Å, the distance between hydrogen and framework oxygen is 3.97 Å. According to the above analysis, our simulation results demonstrated that there are two adsorption sites to have preferential interaction with Na+ located in sites II and III’. Meanwhile, the hydrogen atoms of CHClF2 or CHF3 have intensive interactions with the zeolite framework oxygen. According to the CHClF2 and CHF3 geometric situations formed in the framework supercage, we find that the existence of chlorine further extends the distance between CHClF2 and the framework and the Na-F distance is even farther that of CHF3. It is perceived that CHClF2 and CHF3 demonstrate to have significant difference in binding category and performance. Just

16


Page 16 of 29

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


as what we have observed in the Figures 4a–5a, the Na-F distance shortens as the fluorine concentration for CHF3 increases. This shows the potential reason why the interaction of FAU framework with CHF3 is much more favorable than the interaction with CHClF2. A similar behavior was observed when CHClF2 or CHF3 was adsorbed in 58Al and 88Al FAU zeolite as shown in Figure S3–S6.

(a)

(b)

(c) (d) Figure 4. Typical arrangements of the CHF3 molecules in 96Al model at low ((a) 1 molecule), (b) 8 molecule), intermediate ((c) 16 molecule) and high ((d) 32 molecule) loadings. Color code: gray, C; white, H; blue, F; purple,Na; red, O; yellow, Si.

17



Page 18 of 29

(a)

(b)

(c)

(d)

Figure 5. Typical arrangements of the CHClF2 molecules in 96Al model at low ((a) 1 molecule), (b) 8 molecule), intermediate ((c) 16 molecule) and high ((d) 32 molecule) loadings. Color code: gray, C; white, H; green, Cl; blue, F; purple,Na; red, O; yellow, Si.

Heats of adsorption Just as Figure 6 shows, in FAU zeolite (58Al, 88Al and 96Al), the heats of adsorption were also calculated to the equimolar CHClF2/CHF3 mixture under different partial pressures. With regard to all loadings in both states of infinite dilution and saturation adsorption, the isosteric adsorption heat for CHF3 was 30% higher than the CHClF2 counterparts in 96Al model. A higher isosteric adsorption heat means that the CHF3 molecules will have higher adsorption capability. So, the reason that CHF3 molecules were more competitively was partially because it has higher heat of

18


Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


adsorption. This observation conforms to the CHF3 and CHClF2 geometric situations in the framework supercage displayed in Figure 4a－5a, which show a shorter interaction distance between CHF3 and framework in FAU zeolites with a serious of sodium exchange. In the initial loading period, the heat of adsorption difference between CHF3 and CHClF2 in 96Al zeolite is around 46.6 kJ mol-1 (35.27 kJ mol-1). Our results conform to the growing trend of the monotonic decreasing adsorption heat as the adsorption pressure increases. Similar profiles have already been obtained for 88Al FAU zeolite. According to Table 4 and Table S1, in the models 96Al and 88Al with no occurrence of cation migration, the adsorption heat being calculated is primarily because of the adsorption site binding energy, also contributed by the guest-guest interaction in a small amount. The small Na-O distances changes of the present sites only have slight influence to the adsorption heat. With regard to 58Al model, the heat of adsorption for CHF3 was 20% higher than the CHClF2 counterparts. As Table 3 indicates, the results of our simulation demonstrate that few (but nonzero) cations at site III’ exist in the exposed 58Al model; this preferred binding site adsorbs CHF3 (CHClF2) in the circumstance of low pressure. With the increasing CHF3 (CHClF2) loadings, cations migrate to the supercage, so as to provide more preferred adsorption sites, which are still insufficient for each CHF3 (CHClF2). This accounts for the non-monotonic loading dependence of CHF3 (CHClF2) adsorption heat in the 58Al model shown in Figure 6. In other words, because of the above mentioned reasons, the heat at the beginning is quite large.

19



However, as the preferred sites fill, the heat decreases accordingly, and then increases once again with the migration of the cations. With extremely high CHF3 (CHClF2) loadings, cautions migrate to the supercage, so as to make the cation environment in 58Al supercage become similar to those of 88Al or 96Al, in which, several site III’ cations can act as the adsorption sites for CHF3 (CHClF2) from Table 4. Because of this, under a high pressure, the simulated heats of adsorption in 58Al increase and approach to those of 88Al or 96Al model. As the pressure becomes even higher, the CHF3 (CHClF2) samples higher energy sites constantly, so that its heat of adsorption probably becomes lower. The adsorption heat difference between CHClF2 and CHF3 in all FAU models is significant. Such a difference has been put forward to be responsible for separating the CHF3 and CHClF2 mixed gas. Based on our calculations, we find that under low and high loadings, CHF3 would be bound to the zeolite framework in an more stronger manner than CHClF2. Therefore, CHF3 wins in the competition of the scarce strongly binding sites.

20


Page 20 of 29

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Heat of adsorption for CHClF2 and CHF3 on FAU zeolite at different pressure at 273, 298 and 323 K.

Influences of the Si/Al ratios on FAU-type zeolite It is well-known that the ideal FAU zeolites seldom exist in the natural world. In the practical industries, FAU zeolites usually have different Al contents. Therefore, it is essential to discuss the impacts of the Si/Al ratios to the competitive behavior. Above all, the zeolite models with diversified Si/Al at 298 K had similar adsorption isotherm curves with the ones in 96Al model at 298K in the shape. This has been certified in the supporting information Figure S1 and S2. As a result, there is also a high probability to have various Si/Al ratios at 298 K corresponding to the same adsorption mechanism being proposed in the zeolite of 96Al model. Furthermore, exactly as Figure 7 and S7 indicate, lower Si/Al ratios led to the higher selectivity for CHF3 at 273, 298 and 323 K. For instance, under the adsorption temperature of 298 K

21



and the total pressure of 100 kPa,

Page 22 of 29

96Al had the corresponding highest selectivity for

CHF3 of 40.5 while 58Al had the lowest selectivity for CHF3 of 31.6. As Figure 7 indicated, we could also find the same trend to CHF3/CHClF2 adsorption system at 273 and 323 K. The competitive adsorption mechanism put forward by us can be used to explain. The sodium ions, which were added on these favorable II and III’ sites, are usually believed to be helpful for the selectivity for adsorption of adsorbates. At the same time, in case the equimolecular CHF3 and CHClF2 reached the supercage of the FAU zeolite, on the basis of the competitive adsorption mechanism put forward by us, the possibility for CHF3 being adsorbed to the preferred binding site is much higher. Above all, as the Al content in the zeolite increases, the selectivity of CHF3 increases accordingly. As to FAU zeolites, lower ratios of Si/Al led to better CHF3 adsorptions. That’s why 96Al FAU zeolites was the most effective modeling for CHF3 adsorption with different temperatures. However, theoretically speaking, the Al amount equals to the sodium ions amount in 96Al zeolites. What’s more, sodium ions are corresponding to these adsorption sites. As Ref.

36

indicates,

the higher Al

content is, the more adsorption sites there are. In other words, lower Si/Al ratio brought about more adsorption sites as well. So we can conclude that enhancing a higher Si/Al ratio almost has no use to the adsorption rate. Therefore, although it is indeed a hard work to generate the ideal 96Al FAU zeolites, the adsorption of CHF3 can be promoted through utilizing the adsorbents with low and proper Si/Ai ratios in the process of adsorbing and separating the binary CHF3-CHClF2 mixed gas.

22


Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Selectivity for CHF3 in CHF3/CHClF2 system in 96Al FAU zeolite model at 273, 298 and 323 K. The error of each data was less than 5%. The influences of temperatures From the perspective of the experiment, we could find out that the competitive adsorption behavior of CHF3/CHClF2 system is probably influenced by the adsorption temperature. In this paper, the selectivity for CHF3 is defined by us to describe the influences of temperatures on CHF3 adsorption. With the data being obtained from adsorption isotherms, Eq. (3) was used to calculate the selectivity for CHF3 (S) in the CHF3/CHClF2 systems at different temperatures. Just as Figure 7 indicate, in the circumstances of different temperatures, S always rose and fallen in the beginning. However, as the pressure became higher, it stabilized ultimately. When the S value remained unchanged, it is visible that the S value was lowest at 323 K, which was 14.6. In comparison, the S value is equivalent to around 25.1 in 273 K. It means that when the adsorption system contains CHClF2 molecules, a lower temperature of the adsorption system is helpful for a higher selectivity for CHF3. Due to the higher selectivity for CHF3, we come to a conclusion that the competitive adsorption

23



performance of CHF3 increases as the temperature decreases. It is because, just as Figure 7 shows, CHClF2 has unchanged adsorption amount while CHF3 has increased adsorption amount as the temperature rises. This rule is followed by the FAU-type zeolites with other Si/Al ratios (58Al and 88Al), which is just demonstrated in supporting information Figure S7. In addition, despite of the fact that the selectivity for CHF3 varies as temperature changes, the saturated adsorption capacity of CHClF2 at 273, 298 and 323 K in 96Al FAU zeolite model only had subtle changes, which were merely about 2.82 molecule/unit cell. It indicates that the the selectivity for CHF3 declined merely because of the decreased adsorption amount of CHF3 molecules as the temperature rises. The results show that the low temperature has a positive influence to the selectivity for CHF3. Therefore, in the adsorbing and separating the binary CHF3-CHClF2 mixed gas, a lower temperature is probably helpful to the improvement of selectivity of CHF3 in general adsorption conditions.

Conclusions Sodium cation distributions and geometry structure of CHClF2 and CHF3 in supercage of zeolite has been investigated by MC simulations performed with a new force field. As to the 58Al FAU model, the population of sites II’ and III’ are enhanced because of the adsorption of CHF3 or CHClF2 molecules. A smaller sodium is induced by CHF3 or CHClF2 adsorption to migrate from sites I’ and II to sites II’ and III’. In 88Al and 96Al FAU model, simulation results present no migration of sodium site II and III’ at different CHF3 or CHClF2 loadings. Through the Na–F or Na–Cl interactions, the CHF3 (CHClF2) are matched with two cations in the preferred

24


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sites, in which, one is in site II and the other is in site III’, and by O–H interactions. As to the CHClF2 and CHF3, it is predicted that their loading heat adsorption dependence in FAU-type zeolite is quite different. In normal conditions, CHF3 is bound to the utilized zeolites sample in a stronger manner, because it promotes the close contacts of Na–F and O–H with the zeolites framework oxygen. In addition to this, this paper also employs the GCMC simulations to investigate two practical factors in the competitive adsorption behavior. Lower adsorption temperatures and Si/Al ratios enabled CHF3 to have higher selectivity. This research studies the competitive adsorption mechanism of the CHClF2 and CHF3 in FAU-type zeolite from the perspective of molecular level and reveals the potential of these materials in capturing and separating the CHF3/CHClF2 mixed gas. In addition, it puts forward an efficient and predominant plan in designing and select the adsorbent materials in the field of CHF3–CHClF2 separation and recovery.

Supporting Information Supporting Information associated with this article can be found, in the online version.

Acknowledgements The authors are grateful to Associate Professor Hideki TANAKA of Kyoto University, as well as Professor Minoru MIYAHARA, for their assistance in this work. This research is financially supported by International Academic Exchange Fund of

25



Page 26 of 29

Tianjin University and the State Key Laboratory of Chemical Engineering of Tianjin University (No.SKL-ChE-16B05).

References (1) Corbin, D. R.; Mahler, B. A. Separation of tetrafluoroethane isomers. US005600040A, 1997. (2) Grey, C. P.; Corbin, D. R. 19F and 27Al MAS NMR Study of the Dehydrofluorination Reaction of Hydrofluorocarbon-134 over Basic Faujasite Zeolites. J. Phys. Chem. 1995, 99 (46), DOI 10.1021/j100046a005. (3) Abraham, S.; Giora, A.; Gabriela, B. N.; Dana, B.; Gil, D.; Jack, G.; Lilach, K.; Vitaly, K. Recovery of perfluorinated compouds and hydrofluorocarbon gases using molecular sieve menbrances. EP19980900980, 1998. (4) Lim, K. H.; Grey, C. P.

19

F/23Na Cross polarization NMR study of hydrofluorocarbon–zeolite

binding on zeolite NaY. Chem. Commun. 1998, 0, DOI 10.1039/A805542D. (5) Lim, K. H.; Grey, C. P. Characterization of Extra-Framework Cation Positions in Zeolites NaX and NaY with Very Fast 23Na MAS and Multiple Quantum MAS NMR Spectroscopy. J. Am. Chem. Soc. 2000, 122 (40), DOI 10.1021/ja001281d. (6) Lim, K. H.; Jousse, F.; Auerbach, S. M.; Grey, C. P. Double Resonance NMR and Molecular Simulations of Hydrofluorocarbon Binding on Faujasite Zeolites NaX and NaY: the Importance of Hydrogen Bonding in Controlling Adsorption Geometries. J. Phys. Chem. B 2001, 105 (41), DOI 10.1021/jp0045989. (7) Chatterjee, A.; Ebina, T.; Iwasaki, T.; Mizukami, F., Chlorofluorocarbons adsorption structures and energetic over faujasite type zeolites—a first principle study. J. Mol. Struct. 2003, 630 (1), DOI 10.1016/S0166-1280(03)00156-8. (8) Iijima, S.; Nakamura, M.; Yokoi, A.; Kubota, M.; Huang, L.; Matsuda, H. Decomposition of dichloromethane and in situ alkali absorption of resulting halogenated products by a packed-bed non-thermal

plasma

reactor.

J.

Mater.

Cycles

Waste

Manage.

2011,

13

(3),

DOI

10.1007/s10163-011-0022-0. (9) Ruan, X.; Dai, Y.; Du, L.; Yan, X.; He, G.; Li, B. Further separation of HFC-23 and HCFC-22 by coupling multi-stage PDMS membrane unit to cryogenic distillation. Sep. Purif. Technol. 2015, 156, DOI 10.1016/j.seppur.2015.10.064. (10) UNEP; IEA. IPCC Guidelines for National Greenhouse Gas Inventories; European Environment Agency: Copenhagen, 1995. (11) Blowers, P.; Hollingshead, K. Estimations of Global Warming Potentials from Computational Chemistry Calculations for CH2F2 and Other Fluorinated Methyl Species Verified by Comparison to Experiment. J. Phys. Chem. A 2009, 113 (20), DOI 10.1021/jp8114918. (12) Fang, X.; Miller, B. R.; Su, S.; Wu, J.; Zhang, J.; Hu, J. Historical Emissions of HFC-23 (CHF3) in China and Projections upon Policy Options by 2050. Environ. Sci. Technol. 2014, 48 (7), DOI 10.1021/es404995f. (13) Fu, Q.; Zhou, Y.; Shen, Y.; Yan, H.; Li, D.; Qin, Y.; Zhang, D. R23/R22 Separation and Recovery Using the DIST-PSA Hybrid System. Ind. Eng. Chem. Res. 2017, 56, DOI 10.1021/acs.iecr.6b03701.

26


Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) McCulloch, A.; Midgley, P. M.; Lindley, A. A. Recent changes in the production and global atmospheric emissions of chlorodifluoromethane (HCFC-22). Atmos. Environ. 2006, 40 (5), DOI 10.1016/j.atmosenv.2005.10.015. (15) Luna-Triguero, A.; Vicent-Luna, J. M.; Dubbeldam, D.; Gómez-Álvarez, P.; Calero, S. Understanding and Exploiting Window Effects for Adsorption and Separations of Hydrocarbons. J. Phys. Chem. C 2015, 119 (33), DOI 10.1021/acs.jpcc.5b05597. (16) Jaramillo, E.; Grey, C. P.; Auerbach, S. M. Molecular Dynamics Studies of Hydrofluorocarbons in Faujasite-type Zeolites: Modeling Guest-Induced Cation Migration in Dry Zeolites. J. Phys. Chem. B 2001, 105 (49), DOI 10.1021/jp011997a. (17) Zheng, H.; Zhao, L.; Yang, Q.; Gao, J.; Shen, B.; Xu, C., Influence of Framework Proton s on the Adsorption Sites of the Benzene/HY System. Ind. Eng. Chem. Res. 2014, 53 (35), D OI 10.1021/ie501386f. (18) Torres-Knoop, A.; Heinen, J.; Krishna, R.; Dubbeldam, D., Entropic Separation of Styrene/ Ethylbenzene Mixtures by Exploitation of Subtle Differences in Molecular Configurations in Or dered Crystalline Nanoporous Adsorbents. Langmuir 2015, 31 (12), DOI 10.1021/acs.langmuir.5 b00363. (19) Olson, D. H. The crystal structure of dehydrated NaX. Zeolites 1995, 15 (5), DOI 10.1016/0144-2449(95)00029-6. (20) Haranczyk, M.; Sethian, J. A. Automatic Structure Analysis in High-Throughput Characterization of Porous Materials. J. Chem. Theory Comput. 2010, 6 (11), DOI 10.1021/ct100433z. (21) Haranczyk, M.; Sethian, J. A. Navigating molecular worms inside chemical labyrinths. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (51), DOI 10.1073/pnas.0910016106. (22) Zhu, L.; Seff, K. Cation Crowding in Zeolites. Reinvestigation of the Crystal Structure of Dehydrated Potassium-Exchanged Zeolite X. J. Phys. Chem. B 2000, 104 (38), DOI 10.1021/j p000710r. (23) Beck, L. W.; Xu, T.; Nicholas, J. B.; Haw, J. F. Kinetic NMR and Density Functional Study of Benzene H/D Exchange in Zeolites, the Most Simple Aromatic Substitution. J. Am. Chem. Soc. 1995, 117 (46), DOI 10.1021/ja00151a031. (24) Motkuri, R. K.; Annapureddy, H. V. R.; Vijaykumar, M.; Schaef, H. T.; Martin, P. F.; McGrail, B. P.; Dang, L. X.; Krishna, R.; Thallapally, P. K. Fluorocarbon adsorption in hierarchical porous frameworks. Nat. Commun. 2014, 5, DOI 10.1038/ncomms5368. (25) Di Lella, A.; Desbiens, N.; Boutin, A.; Demachy, I.; Ungerer, P.; Bellat, J. P.; Fuchs, A. H. Molecular Simulation Studies of Water Physisorption in Zeolite. Phys. Chem. Chem. Phys. 2006, 8, DOI 10.1039/B610621H. (26) Goel, H.; Butler, C. L.; Windom, Z. W.; Rai, N. Vapor Liquid Equilibria of Hydrofluorocarbons Using Dispersion-Corrected and Nonlocal Density Functionals. J. Chem. Theory Comput. 2016, 12 (7), DOI 10.1021/acs.jctc.6b00305. (27) Mountain, R. D.; Morrison, G. Molecular dynamics study of liquid CCl2F2 and CCIHF2. Mol. Phys. 1988, 64 (1), DOI 10.1080/00268978800100083. (28) Calero, S.; Dubbeldam, D.; Krishna, R.; Smit, B.; Vlugt, T. J.; Denayer, J. F.; Martens, J. A.; Maesen, T. L. Understanding the role of sodium during adsorption: a force field for alkanes in sodium-exchanged faujasites. J. Am. Chem. Soc. 2004, 126 (36), DOI 10.1021/ja0476056. (29) Feuerstein, M.; Hunger, M.; Engelhardt, G.; Amoureux, J. P. Characterisation of sodium cations in dehydrated zeolite NaX by 23Na NMR spectroscopy. Solid State Nucl. Magn. Reson. 1996, 7 (2), DOI

27



10.1016/S0926-2040(96)01246-5. (30) Fitch, A. N.; Jobic, H.; Renouprez, A., Localization of benzene in sodium-Y-zeolite by powder neutron diffraction. J. Phys. Chem. 1986, 90 (7), DOI 10.1021/j100398a021. (31) Newsam, J. M.; Jacobson, A. J.; Vaughan, D. E. W. Time-of-flight powder neutron diffraction studies of dehydrated synthetic gallosilicate zeolites X, XY, and Y. J. Phys. Chem. 1986, 90 (26), DOI 10.1021/j100284a030. (32) Van Dun, J. J.; Dhaeze, K.; Mortier, W. J. Temperature-dependent cation distribution in zeolites. 2. Dehydrated NaxHY (x = 13, 23, 42, 54), Ca15HY, and Sr27Y. J. Phys. Chem. 1988, 92 (23), DOI 10.1021/j100334a051. (33) Vitale, G.; Mellot, C. F.; Bull, L. M.; Cheetham, A. K. Neutron Diffraction and Computational Study of Zeolite NaX: Influence of SIII′ Cations on Its Complex with Benzene. J. Phys. Chem. B 1997, 101 (23), DOI 10.1021/jp970393x. (34) Buttefey, S.; Boutin, A.; Mellot-Draznieks, C.; Fuchs, A. H. A Simple Model for Predicting the Na+ Distribution in Anhydrous NaY and NaX Zeolites. J. Phys. Chem. B 2001, 105 (39), DOI 10.1021/jp0105903. (35) Jaramillo, E.; Auerbach, S. M. New Force Field for Na Cations in Faujasite-Type Zeolites. J. Phys. Chem. B 1999, 103 (44), DOI 10.1021/jp991387z. (36) Frising, T.; Leflaive, P. Extraframework cation distributions in X and Y faujasite zeolites: A review. Microporous Mesoporous Mater. 2008, 114, DOI 10.1016/j.micromeso.2007.12.024.

28


Page 28 of 29

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract Graphic

Synopsis Investigating CHF3–CHClF2 binary mixture competitive adsorption mechanism in FAU-type zeolite to separate and recover the greenhouse gases from emissions, can reduce adverse environmental and economic footprints.

29


Competitive Adsorption Mechanism Study of CHClF2 and CHF3 in

Recommend Documents